The present invention relates to a method of producing an electrode for non-aqueous electrolytic cells, which is improved in charge and discharge characteristics such as discharge potential, discharge capacity, and charge and discharge cycle life.
As electrodes for non-aqueous electrolytic cells high in discharge potential and discharge capacity, lithium ion secondary cells occluding and releasing lithium have recently come in practice. In the production of these cells, paints for active substance layers are prepared using active substances and binders, and applied onto collectors, followed by drying. In the procedure, the composition and compounding of the materials, and production conditions such as kneading, dispersing and coating are very important. The settings thereof largely change the cell characteristics produced. Even when the composition and compounding are optimized, the characteristics are sometimes not exhibited depending on the production conditions. Accordingly, the conditions have successively been studied.
Inventions in which such production conditions have been studied are ones described in Japanese Patent Unexamined Publication Nos. 4-253157 and 7-29605. In Japanese Patent Unexamined Publication No. 4-253157, it is described that materials are efficiently mixed by using a mixer of the form that a stirring rod revolves in a kneading vessel while rotating on its axis. Further, in Japanese Patent Unexamined Publication No. 7-29605, kneading with a kneader is studied for the necessity to more enhance kneading dispersion, for increasing the weight of an electrode mixture in the specified volume, and for increasing the filling density of the mixture.
Furthermore, Japanese Patent Unexamined Publication No. 6-203833 discloses an electrode for an organic electrolytic cell in which a mixture of an insoluble, infusible substrate having a polyacene skeleton structure (PAS), a kind of polymer carbon, as an electrode active substance, a fluorine-containing polymer and polyvinylpyrrolidone is used.
However, the mixing type disperser according to the invention described in Japanese Patent Unexamined Publication No. 4-253157 or 7-29605, in which the stirring rod revolves and rotates on its axis, also has spaces other than a kneading portion. Accordingly, the materials escape into the spaces to cause great kneading loss, resulting in failure to achieve sufficient kneading. Also an open type kneader according to the invention described in Japanese Patent Unexamined Publication No. 7-29605 mentioned above is used, the materials escape similarly into the spaces, resulting in insufficient dispersion. A pressurization type kneader has the problem that the dispersibility is entirely different depending on the compounded state of the materials. In particular, when graphite showing a flake form is used as a conductive material for improving the charge and discharge cycle durability by lowering the internal resistance while increasing the filling amount of an active substance, this tendency becomes significant because the material form thereof is largely different from that of the active substance.
Further, there is a problem that it is impossible to take advantage of the characteristic, the high capacity, of polymer carbon when the composition compounding according to the invention described in Japanese Patent Unexamined Publication No. 6-203833 is only employed.
In view of the problems as described above, the invention provides a production method which can take advantage of the material characteristics when an active substance composed of polymer carbon, a conductive agent composed of graphite showing a flake form, and a binder are used.
In order to solve the above-mentioned problems, the present inventors have conducted intensive investigation. As a result, the inventors have discovered that the above-mentioned problems can be solved to achieve an object of the invention by a method of producing an electrode for non-aqueous electrolytic cells comprising forming on a collector an electrode active substance layer comprising an active substance composed of polymer carbon, a conductive agent composed of graphite showing a flake form, and a binder, in which the specific surface area and compounded weight of the active substance, and the specific surface area and compounded weight of graphite satisfy a specific equation, thus completing the invention.
That is to say, the invention provide (1) a method of producing an electrode for non-aqueous electrolytic cells comprising forming on a collector an electrode active substance layer comprising an active substance composed of polymer carbon, a conductive agent composed of graphite showing a flake form, and a binder, characterized in that when the specific surface area of the above-mentioned active substance as measured by the BET method is denoted by Sa, the compounded weight thereof at the time of kneading by Wa, the specific surface area of the above-mentioned graphite as measured by the BET method by Sg, and the compounded weight thereof at the time of kneading by Wg, the above-mentioned mixture is kneaded by a kneading device under conditions satisfying the following equation:
Wa/Wgxe2x89xa70.5xc2x7Sg/Sa;
(2) the method of producing an electrode for non-aqueous electrolytic cells described in the above (1), wherein the polymer carbon has a polyacene skeleton structure, the hydrogen atom/carbon atom (H/C) is from 0.05 to 0.5, and the specific surface area as measured by the BET method is from 0.1 to 20 m2/g; and (3) the method of producing an electrode for non-aqueous electrolytic cells described in the above (1), wherein the specific surface area of the graphite as measured by the BET method is from 1 to 30 m2/g.